Hybrid phase lock loop

ABSTRACT

Hybrid phase lock loop (PLL) devices are provided that combine advantages of the digital controlled loop and the analog controlled loop. For example, a hybrid PLL includes a digital controlled loop that receives a reference input signal and an output signal of the hybrid PLL, and generates a digital tuning word. The hybrid PLL further includes an analog controlled loop that receives the reference input signal and the output signal of the hybrid PLL, and generates an output voltage. The hybrid PLL also includes a hybrid oscillator. An oscillator controller of the digital controlled loop controls the hybrid oscillator using the digital tuning word and disables the analog controlled loop during a frequency tracking operational mode of the hybrid PLL. The oscillator controller enables the analog controlled loop to control the hybrid oscillator during the phase tracking operational mode of the hybrid PLL.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/189,482, filed on Nov. 13, 2018, which is a continuation ofU.S. patent application Ser. No. 15/428,841, filed on Feb. 9, 2017, nowU.S. Pat. No. 10,164,649 which claims the benefit of U.S. ProvisionalPatent Application No. 62/428,146, filed on Nov. 30, 2016, each of whichis hereby incorporated by reference in its entirety.

BACKGROUND

This disclosure generally relates to phase lock loop circuits.

A phase lock loop (PLL) circuit is an electronic control circuit thatgenerates an output clock signal having a phase that is locked to thephase of an input reference signal. For example, a PLL can be used toadjust an oscillator so that a frequency and phase of a signal generatedby the oscillator matches the frequency and phase of a reference inputsignal. A PLL circuit is commonly used in communication devices,computers, and other electronic devices. An analog PLL circuit usesanalog components to provide the phase lock architecture. These analogcomponents include a phase detector, a voltage-controlled oscillator(VCO), and a feedback path between the VCO output signal and an inputport of the phase detector. By connecting the input reference signal toanother input port of the phase detector, the output of the phasedetector may be used to adjust the phase and/or the frequency of the VCOoutput signal until that phase and/or frequency is locked to the inputreference signal.

A PLL circuit may also be implemented using all digital components. Sucha PLL circuit is known as an all-digital PLL (ADPLL) circuit. Like itsanalog counterpart, an ADPLL circuit uses a feedback path to return adigitally-controlled oscillator (DCO) clock signal to generate a digitalphase error signal based on the output from a time-to-digital converter(TDC) and a reference phase signal. In response to the digital phaseerror signal, the phase of the DCO clock signal is adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of this disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the common practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram of an exemplary hybrid PLL, according to anembodiment of the present disclosure

FIG. 2 illustrates timing diagrams for an exemplary hybrid PLL,according to an embodiment of the present disclosure.

FIG. 3A is an exemplary implementation of a hybrid oscillator that canbe implemented with a hybrid PLL, according to an embodiment of thepresent disclosure.

FIG. 3B is a frequency arrangement, according to an embodiment of thepresent disclosure.

FIG. 4A is another exemplary implementation of a hybrid oscillator thatcan be implemented with a hybrid PLL, according to an embodiment of thepresent disclosure.

FIG. 4B is an exemplary implementation of a current controlledoscillator (CCO) that can be implemented with a hybrid PLL, according toan embodiment of the present disclosure.

FIG. 5 is a more detailed implementation of a hybrid PLL, according toan embodiment of the present disclosure.

FIG. 6 depicts diagrams that illustrate trend detection, according tosome embodiments of the disclosure.

FIG. 7 illustrates simulated waveforms, according to an embodiment ofthe disclosure.

FIG. 8 is a flowchart illustrating an exemplary operational controlflow, according to an embodiment of this disclosure.

Illustrative embodiments will now be described with reference to theaccompanying drawings. In the drawings, like reference numeralsgenerally indicate identical, functionally similar, and/or structurallysimilar elements.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the presentdisclosure may repeat reference numerals and/or letters in the variousexamples. This repetition does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

It is noted that references in the specification to “one embodiment,”“an embodiment,” “an example embodiment,” “exemplary,” etc., indicatethat the embodiment described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases do not necessarily refer to the same embodiment. Further,when a particular feature, structure or characteristic is described inconnection with an embodiment, it would be within the knowledge of oneskilled in the art to effect such feature, structure or characteristicin connection with other embodiments whether or not explicitlydescribed.

It is to be understood that the phraseology or terminology herein is forthe purpose of description and not of limitation, such that theterminology or phraseology of the present specification is to beinterpreted by those skilled in relevant art(s) in light of theteachings herein.

The term “about” as used herein indicates the value of a given quantityvaries by ±10% of the value, unless noted otherwise.

A phase lock loop (PLL), such as a hybrid phase lock loop to provide anexample, of the present disclosure can be used to adjust its oscillatorso that a frequency and/or a phase of an output signal generated by theoscillator is proportional to a frequency and/or a phase of a referenceinput signal. The PLL includes a phase and/or frequency detector thatprovides an error signal representing a difference, in frequency and/orphase, between the output signal and the reference input signal. Thiserror signal can be measured to ensure that the frequency and/or thephase of the output signal is proportional to the frequency and/or thephase of the reference signal. For example, as the PLL adjusts theoscillator, the frequency and/or the phase of the output signal cangradually become closer to the frequency of the reference input signal.When the frequency and the phase of the output signal is proportional tothe frequency and/or the phase of the reference input signal, the PLL issaid to be locked onto the reference input signal. The time it takes forthe frequency and/or phase of the output signal to become proportionalto the frequency and/or the phase of the reference input signal can bereferred to as the locking time.

In an embodiment, the hybrid PLL of this disclosure operates in afrequency tracking mode to adjust the frequency of the output signal tobe proportional to a frequency of the reference input signal, or, in aphase tracking mode to adjust a phase of the output signal to match anyvariations in the reference input signal. The frequency tracking mode isperformed by a digital controlled loop of the hybrid PLL. The digitalcontrolled loop (DCL) of the hybrid PLL can provide fast tracking forreducing the locking time. On the other hand, the phase tracking mode isperformed by an analog controlled loop of the hybrid PLL. The analogcontrolled loop (ACL) of the hybrid PLL can provide very little, or no,quantization noise at steady state. By combining the DCL and ACL, thehybrid PLL can solve the problems with the large gain of avoltage-controlled oscillator (VCO) of an analog PLL (APLL), and thequantization noise caused by an all-digital PLL (ADPLL)

FIG. 1 is a block diagram of an exemplary hybrid PLL 100 according to anembodiment of the present disclosure. A reference input signal 101represents a first time-varying signal, such as a sine wave to providesome examples, having a frequency f_(REF) and a phase ϕ_(REF).Similarly, an output signal 151 represents a second time-varying signalhaving a frequency f_(OUT) and a phase ϕ_(OUT). Herein, the frequencyf_(REF) and the phase ϕ_(REF) of the first time-varying signal isreferred to as the f_(REF) and the phase ϕ_(REF), respectively.Similarly, the frequency f_(OUT) and the phase ϕ_(OUT) of the secondtime-varying signal is referred to as the f_(OUT) and the phase ϕ_(OUT),respectively. Hybrid PLL 100 adjusts output signal 151 such that thefrequency f_(OUT) and/or the phase ϕ_(OUT) is proportional to thefrequency f_(REF) and/or the phase ϕ_(REF). Hybrid PLL 100 can operatein the frequency tracking mode to adjust the frequency f_(OUT) to beproportional to the frequency f_(REF) or in the phase tracking mode toadjust the phase ϕ_(OUT) to match, or substantially match, the phaseϕ_(REF).

Hybrid PLL 100 can be implemented using a frequency detector 111, adigital loop filter 113, an oscillator controller 115, a feedbackdivider 131, a phase frequency detector (PFD) 133, a charge pump andanalog filter 135, and a hybrid oscillator 153.

The components of hybrid PLL 100 can be divided in three portions—DCL110, ACL 130, and oscillator circuit 150. DCL 110 includes frequencydetector 111, digital loop filter 113, and oscillator controller 115 andis configured to perform the frequency tracking mode of hybrid PLL 100.The components of DCL 110 can be implemented using digital components.DCL 110 is configured to generate a digital signal (digital tuning word107) that controls hybrid oscillator 153 during the frequency trackingoperation mode of the hybrid PLL.

In one embodiment, ACL 130 includes feedback driver 131, phase frequencydetector 133, and charge pump and analog filter 135 and is configured toperform the phase tracking mode of hybrid PLL 100. The components of ACL130 can be implemented using analog components or using analog anddigital components. For example, charge pump and filter 135 of ACL 130can be implemented using analog components; and phase frequency detector133 and feedback divider 131 of ACL 130 can be implemented using analogand/or digital components. ACL 130 is configured to generate an analogsignal (output voltage 121 (VCOIN)) that controls hybrid oscillator 153during the phase tracking operation mode of the hybrid PLL.

In this embodiment, oscillator circuit 150 of PLL 100 includes hybridoscillator 153 and is configured to provide discrete frequency tuning inthe frequency tracking mode and continuous frequency tuning in the phasetracking mode. In one example, hybrid oscillator 153 is implementedusing analog components. The components of oscillator circuit 150 can beimplemented using analog components. Alternatively, ACL 130 can includefeedback divider 131 and phase frequency detector 133. In this example,charge pump and analog filter 135 can be included in oscillator circuit150 with hybrid oscillator 153. In one embodiment of this example,oscillator circuit 150 can be implemented using analog components.

Frequency detector 111 receives input signal 101 and output signal 151.Frequency detector 111 compares the frequency f_(OUT) of output signal151 and the frequency f_(REF) of input signal 101 to provide an errorsignal 103. In one example, frequency detector 111 is configured toconvert at least one of output signal 151 and input signal 101 intodigital signals before comparing the frequencies. Frequency detector 111provides digital error signal 103. If error signal 103 is zero or closeto zero, it indicates that the frequency f_(OUT) of output signal 151 istracking and/or is a multiple of the frequency f_(REF) of input signal101.

Digital loop filter 113 is configured to control the bandwidth of hybridPLL 100 in DCL mode. Digital loop filter 113 receives error signal 103.Error signal 103 represents a digital representation of a thirdtime-varying signal. In one example, digital loop filter 113 suppresseshigh frequency components in the third time-varying signal which areoutside of its bandwidth to provide samples of a direct current (DC), ornear DC, component of the third time-varying signal within its bandwidthas the signal 105.

Oscillator controller 115 receives signal 105 from digital loop filter113 and reference input signal 101. Oscillator controller 115 isconfigured to analyze signal 105 and reference input signal 101 andgenerate digital tuning word 107 and tracking signal 109. During thefrequency tracking mode where hybrid PLL 100 is using DCL 110, digitaltuning word 107 is used to control the frequency of hybrid oscillator153. In one example, oscillator controller 115 generates digital tuningword 107 by multiplying signal 105 by a normalization value(NROM_(RO)=f_(REF)/Gain_(RO)) to provide process effect cancellation. Inone example, the tolerance of the normalization value for DCL can reacharound ±100%. Also, an incorrect normalization value can affect onlytracking time and not stability of hybrid PLL 100. Oscillator controller115 can also include a function of retiming to avoid frequency glitcheswhen digital tuning words transition. Digital tuning word 107 is aninput signal to hybrid oscillator 153 to tune the frequency of hybridoscillator 153. DCL 110 of hybrid PLL 100 performs the frequencytracking mode by tuning digital tuning word 107. In one example, digitaltuning word 107 can be a 5-bit binary code, which can provide a widefrequency tuning range.

In addition to generating digital tuning word 107, oscillator controller115 controls the overall configuration and operation of hybrid PLL 100.Oscillator controller 115 configures hybrid PLL 100 to operate thefrequency tracking mode using DCL 110. In the frequency tracking modeusing DCL 110, oscillator controller 115 disables tracking signal 109 bysetting tracking signal 109 to a first logical level (e.g., low logicallevel.) Hybrid oscillator 153 adjusts output signal 151 to adjust thefrequency f_(OUT) in the frequency tracking mode based on the receiveddigital tuning word 107. Thereafter, oscillator controller 115 monitorssignal 105 (which is created based on error signal 103) to determine acondition to switch to phase tracking mode. Once oscillator controller115 detects the switching condition, oscillator controller 115configures hybrid PLL 100 to operate in the phase tracking mode usingACL 130 (e.g., enables ACL 130.) The switching condition indicates thefrequency f_(REF) is sufficiently close to the frequency f_(OUT) toallow hybrid oscillator to lock onto reference input signal 101 in thephase tracking mode of operation using ACL 130.

In the phase tracking mode of operation using ACL 130, oscillatorcontroller 115 enables tracking signal 109 by setting tracking signal109 to a second logical level (e.g., high logical level.) Hybridoscillator 153, using ACL 130, adjusts output signal 151 to adjust thephase ϕ_(OUT) in the phase tracking mode. When the phase component ofthe error signal output of phase frequency detector 133 is minimized,the phase ϕ_(OUT) is sufficiently close to the phase ϕ_(REF). In thissituation, the hybrid oscillator is locked onto the reference inputsignal 101 so as to track any variations in the frequency f_(REF) andthe phase ϕ_(REF).

According to one example, oscillator controller 115 is configured todetermine the switching condition by monitoring the signal 105 (which iscreated based on error signal 103) to determine a trend, for example, apositive trend, a flat trend, and/or a negative trend in signal 105. Anexample of this trend detection process is discussed in U.S. patentapplication Ser. No. 15/135,212, filed Apr. 5, 2016 and titled“Automatic Detection of Change in PLL Locking Trend,” which isincorporated by reference in its entirety. The positive trend indicatesa digital value of signal 105 is increasing from a previous value ofsignal 105, the flat trend indicates the digital value of signal 105 issubstantially unchanged from the previous value of signal 105, and thenegative trend indicates the digital value of signal 105 is decreasingfrom the previous value of signal 105. Once oscillator controller 115detects a first change in the trend of signal 105, for example, from thepositive trend to the flat trend or the negative trend to the flattrend, oscillator controller 115 configures hybrid PLL 100 to use ACL130 to operate in the phase tracking mode.

Similarly, while hybrid PLL 100 is configured to use ACL 130 to operatein the phase tracking mode, oscillator controller 115 is configured tomonitor the error signal 103 to determine whether a second change in thetrend of the error signal occurs. The second change in trend can includea change from the flat trend to the positive trend or from a flat trendto the negative trend. The second change in the trend can indicate thatthe frequency of the output signal 151 is no longer sufficiently closeto the frequency of the reference input signal 101. If oscillatorcontroller 115 detects a second change in the trend of error signal 103,oscillator controller 115 configures hybrid PLL 100 to use DCL 110 tooperate again in the frequency tracking mode.

According to some embodiments, ACL 130 includes feedback divider 131,phase frequency detector 133, and charge pump and analog filter 135.Before hybrid PLL 100 enters the phase tracking mode using ACL 130,tracking signal 109 is set to the first logical level (e.g., low logicallevel) and output voltage 121 (VCOIN) is set to a fixed voltage (e.g.,VDD/2). Hybrid PLL 100 starts operating in the phase tracking mode usingACL 130 when oscillator controller 115 enables tracking signal 109 bysetting tracking signal 109 to a second logical level (e.g., highlogical level.)

Feedback divider 131 is a synchronous high speed divider driven byoutput signal 151. Feedback divider 131 receives output signal 151 andgenerates feedback signal 123. Phase frequency detector 133 receivesfeedback signal 123 and reference input signal 101. Phase frequencydetector 133 is configured to detect the phase difference (and/orfrequency difference) between feedback signal 123 and reference inputsignal 101. Phase frequency detector 133 generates two output signalswith narrow pulse width (e.g., around 40 ps)—UP signal 125 and DN signal127. The pulse signals UP signal 125 and DN signal 127 are input tocharge pump and analog filter 135. Although FIG. 1 is described with thephase frequency detector 133, it is noted that other phasedetectors/comparators can also be used.

Charge pump and analog filter 135 can include a charge pump and a loopfilter. The charge pump of charge pump and analog filter 135 convertsthe UP signal 125 and DN signal 127 to a corresponding UP/DN current.The analog filter of charge pump and analog filter 135 converts theUP/DN current output of the charge pump into the output voltage 121(VCOIN). Output voltage 121 (VCOIN) is input to hybrid oscillator 153.As discussed above, during the frequency tracking mode using DCL 110 ofhybrid PLL 100, tracking signal 109 (which is input to charge pump andanalog filter 135) is set to the first logical level (e.g., low logicallevel) and therefore, output voltage 121 (VCOIN) is set to a fixedvoltage (e.g., VDD/2). When hybrid PLL 100 starts operating in the phasetracking mode using ACL 130, tracking signal 109 is set to a secondlogical level (e.g., high logical level) and output voltage 121 (VCOIN)is used for phase tracking. During the phase tracking mode using ACL130, digital tuning word 107 is frozen.

Hybrid oscillator 153 is configured to receive digital tuning word 107and output voltage 121 (VCOIN) and generate output signal 151. Asdiscussed above, digital tuning word 107 (e.g., a 32 thermometer code)is used during the frequency tracking mode using DCL 110. Using digitaltuning word 107 generated by DCL 110, hybrid oscillator 153 adjusts thefrequency f_(OUT) of output signal 151 to be in a range that issufficiently close, or closer than before tuning, to the frequencyf_(REF) of reference input signal 101. On the other hand, output voltage121 (VCOIN) is used during the phase tracking mode using ACL 130. In oneexample, output voltage 121 (VCOIN) is a continuous voltage in thetuning range of about 0.2 volts or 0.3 volts to VDD (e.g., the coresupply voltage). Hybrid oscillator 153 fine tunes the frequency f_(OUT)of output signal 151 and adjusts the phase ϕ_(OUT) of output signal 151in accordance with output voltage 121 (VCOIN) generated by ACL 130 totrack the frequency f_(REF) and the phase ϕ_(REF) of reference inputsignal.

FIG. 2 illustrates timing diagrams for hybrid PLL 100, according to someembodiments of present disclosure. Graph 200 illustrates the frequencyf_(OUT) of output signal 151 versus time. As illustrated in graph 200,in the frequency tracking mode of graph 200, hybrid PLL 100 uses DCL 110to track the frequency f_(REF) of reference input signal 101. In thephase tracking mode of graph 200, hybrid PLL 100 uses ACL 130 to trackthe phase ϕ_(REF) of reference input signal 101. The phase tracking modealso includes fine tuning the frequency f_(OUT) of output signal 151such that hybrid PLL 100 tracks the frequency f_(REF) and the phaseϕ_(REF) of reference input signal 101.

Graph 210 illustrates digital tuning word 107 versus time. During thefrequency tracking mode of graph 210, digital tuning word 107 isadjusted by DCL 110 based on the difference between the frequencyf_(OUT) of output signal 151 and the frequency f_(REF) of referenceinput signal 101. During the frequency tracking mode, the frequencyf_(OUT) of output signal 151 illustrated in graph 200, follows digitaltuning word 107 as illustrated in graph 210. When the switch condition(switch between the frequency tracking mode of operation and the phasetracking mode of operation) is met, hybrid PLL 100 switches to the phasetracking mode using ACL 130. During the phase tacking mode of graph 210,digital tuning word 107 is fixed.

Graph 220 illustrates output voltage 121 (VCOIN) versus time. During thefrequency tracking mode of operation, charge pump and analog filter 135is disabled. Therefore, output voltage 121 (VCOIN) is kept at a fixedvoltage. In one exemplary embodiment, the fixed voltage can be half ofVDD (e.g., 0.38 volts.) However, the embodiments of this disclosure arenot limited to this value. When hybrid PLL 100 switches from thefrequency tracking mode using DCL 110 to the phase tracking mode usingACL 130, ACL 130 and charge pump and analog filter 135 are enabled.Accordingly, output voltage 121 (VCOIN) is adjusted by ACL 130 based onthe difference between the phase ϕ_(OUT) of output signal 151 and thephase ϕ_(REF) of reference input signal 101. During the phase trackingoperation mode, the output voltage of the analog controlled loop (VCOIN121) tracks phase of the reference input signal.

Graph 230 illustrates tracking signal 109 versus time. In the frequencytracking mode of operation, tracking signal 109 is disabled by being setto the first logical level (e.g., low logical level.) In the phasetracking mode using ACL 130, oscillator controller 115 enables trackingsignal 109 by setting tracking signal 109 to the second logical level(e.g., high logical level.) Graph 240 illustrates the locking time, whenhybrid PLL 100 is locked onto the reference input signal. Lock occurswhen the frequency and the phase of the output signal is proportional tothe frequency and/or the phase of the reference input signal.

FIG. 3A illustrates an exemplary implementation of a hybrid oscillatorthat can be implemented with hybrid PLL 100, according to an embodimentof the present disclosure. In one example, hybrid oscillator 300 of FIG.3A can be an implementation of hybrid oscillator 153 of FIG. 1. Hybridoscillator 300 can be implemented using a digital tuning bank (DTB) 301,an analog tuning bank (ATB) 303, a current mirror 305, and a currentcontrolled oscillator 307.

Digital tuning bank 301 includes one or more current sources and one ormore switches. Each current source of digital tuning bank 301 isconnected to a switch in series that constructs one digital tuning bitof digital tuning bank 301. The digital tuning bits are connected toeach other in parallel. The switch of each digital tuning bit of bank301 is controlled by the digital tuning word. The digital tuning wordcan include digital tuning word 107 of FIG. 1. Digital tuning bank 301can be configured to provide a wide frequency tuning range. According toone example, the frequency tuning range of the digital tuning bank 301can be divided into 32 steps using 32 of 5-bits binary codes. Forexample, a frequency range of 2.4 GHz can be divided into 32 steps eachwith a step size of 75 MHz. It is noted that other frequency rangesand/or other number of steps can also be used. According to one exampledigital tuning word 107 can be a thermometer code that controls digitaltuning bank 301. A thermometer code can represent a natural number, N,with N ones followed by a zero (if the natural number is understood asnon-negative integer) or with N−1 ones followed by a zero (if naturalnumber is understood as strictly positive integer). In this example, thenumber of current sources of digital tuning bank 301 can be equal to thenumber of steps in the frequency range.

Analog tuning bank 303 can be implemented by a metal-oxide-semiconductorfield-effect transistor (MOSFET) such as, but not limited to, an-channel MOSFET. In one example, the transistor of analog tuning bank303 has a gate terminal that receives the analog signal output voltage121 (VCOIN) of FIG. 1. In this example, the source terminal of thetransistor of analog tuning bank 303 can be coupled to a low voltage(such as ground) and also to digital tuning bank 301. The drain terminalof the transistor of analog tuning bank 303 can be coupled to currentmirror 305 and also to digital tuning bank 301. Analog tuning bank 303is configured to operate during the phase tracking mode of the hybridPLL 100 using ACL 130 and is configured to convert output voltage 121(VCOIN) to an output current. Analog tuning bank 303 provides acontinuous and fine tuning mechanism to the hybrid PLL 100. In oneexample (as discussed in more detail with respect to FIG. 3B), analogtuning bank 303 provides a 750 MHz tuning range for a 0.25 volt tuningvoltage. ACL 130 and analog tuning bank 303 perform the phase trackingmode of the hybrid PLL 100, and therefore, very small or no quantizationnoise is introduced in the steady state operation of the hybrid PLL 100as usually seen in ADPLLs. It is noted that the illustrated analogtuning bank 303 is presented as an example, and other analog tuning bankcircuits can also be used.

Hybrid oscillator 300 further includes a current mirror 305. On oneside, current mirror 305 is coupled to digital tuning bank 301 andanalog tuning bank 303. On the other side, current mirror 305 is coupledto current controlled oscillator (CCO) 307. Current mirror 305 isconfigured to combine the currents of digital tuning bank 301 and analogtuning bank 303 and to drive CCO 307. In a non-limiting example, currentmirror 305 can include two p-channel MOFSETs 309 and 311. In thisexample, source terminals of transistors 309 and 311 are coupled to VDD.Drain and gate terminals of transistor 309 are coupled to each other andalso coupled to digital tuning bank 301 and analog tuning bank 303(e.g., to drain of the transistor of analog tuning bank 303.) The gateterminals of transistors 309 and 311 are coupled to each other, and thedrain terminal of transistor 311 is coupled to CCO 307. It is noted thatthe implementation of current mirror 305 in FIG. 3A is an exemplaryimplementation and other implementations (e.g., active current mirror,high power supply rejection ratio (PSRR) active current mirror,wide-swing current mirrors, Wilson current mirror, etc.) also can beused.

Hybrid oscillator 300 also includes current controlled oscillator (CCO)307. CCO 307 can include a ring oscillator and can be implemented usingsingle-ended or differential multi-stages. Although CCO 307 isillustrated as a single-ended ring oscillator with five stages, theembodiments of this disclosure are not limited to this example and otherCCOs can be used. CCO 307 is coupled to current mirror 305. Currentmirror 305 is configured to control CCO to generate output signal 151 bycontrolling the amount of current supplied to CCO 307. Generally, thefrequency of CCO 307 will increase with increasing current supply fromcurrent mirror 305. When hybrid PLL 100 operates in the frequencytracking mode using DCL 110, digital tuning word 107 controls digitaltuning bank 301, which in turn controls CCO 307 through current mirror305. When hybrid PLL 100 operates in the phase tracking mode using ACL130, output voltage 121 (VCOIN) controls analog tuning bank 303, whichin turn controls CCO 307 through current mirror 305.

FIG. 3B illustrates a frequency arrangement 340, according to anembodiment of the present disclosures. Frequency arrangement 340 of FIG.3B illustrates a frequency range 342 that is covered by digital tuningbank 301 of FIG. 3A and a frequency range 344 covered by analog tuningbank 303 of FIG. 3A. It is noted that the frequency arrangement 340 isan exemplary arrangement and any other frequency arrangement can beused.

In the exemplary arrangement of FIG. 3B, the digital tuning bank 301 canhave a frequency range of 2.4 GHz. In this example, the step size ofdigital tuning bank is determined by the frequency range of analogtuning bank divided by a constant (e.g., 10 in this example.) Theconstant 10 in this example is a covering ratio of frequency range ofanalog tuning bank to step size of digital tuning bank. In other words,the frequency range of analog tuning bank covers 10 times of a step sizeof digital tuning bank. In the example of FIG. 3B, the frequency range2.4 GHz of digital tuning bank is divided into 32 steps (e.g.,thermometer code input to hybrid oscillator 115.) Therefore, each stepin the frequency range of digital tuning bank is a 75 MHz step size.Each step in the frequency range of digital tuning bank corresponds to afraction of the frequency range of the analog tuning bank. Consideringthe constant 10 discussed above, the frequency range of analog tuningbank would be 750 MHz. Given a voltage range of 0.25 volts (from 0.2volts to 0.45 volts) for the analog tuning bank, the VCO gain of theanalog tuning bank would be 3 GHz/V.

The frequency arrangement of FIG. 3B can be summarized in threeequations. The first equation is that the frequency range of analogtuning bank is equal to the gain of VCO multiplied by the voltage range.The second equations is that the step size of digital tuning bank isequal to the frequency range of analog tuning bank divided by a constant(e.g., a covering ratio—for example 10 in the example above.) The thirdequation is that the number of steps of the digital tuning bank is equalto the frequency range of digital tuning bank divided by a step size ofdigital tuning bank. In one example, gain of VCO, voltage range, andfrequency range of digital tuning bank are known values, from, forexample, design specifications. Accordingly, the frequency range of theanalog tuning bank, the step size of the digital tuning bank, and thenumber of steps of the digital tuning bank can be calculated using theequations discussed above.

FIG. 4A illustrates another exemplary implementation of a hybridoscillator that can be implemented with hybrid PLL 100, according to anembodiment of the present disclosure. In one example, hybrid oscillator400 of FIG. 4A can be an implementation of hybrid oscillator 153 ofFIG. 1. Hybrid oscillator 400 can be implemented using a digital tuningbank (DTB) 401, an analog tuning bank (ATB) 403, an active currentmirror 405, and a current controlled oscillator (CCO) 407. Digitaltuning bank 401, analog tuning bank 403, and current controlledoscillator 407 of hybrid oscillator 400 are similar to digital tuningbank 301, analog tuning bank 303 and a current controlled oscillator 307of hybrid oscillator 300 of FIG. 3A, and therefore, are not discussedseparately.

Hybrid oscillator 400 of FIG. 4A is further implemented using activecurrent mirror 405. Current mirror 305 of hybrid oscillator 300 of FIG.3A is replaced by active current mirror 405. In one embodiment, activecurrent mirror 405 can include two p-channel MOFSETs, an amplifier, anda resistor-capacitor circuit (RC circuit). According to one example,using hybrid oscillator 400 can enhance power supply rejection ration(PSRR) of the hybrid PLL.

FIG. 4B illustrates an exemplary implementation of a current controlledoscillator (CCO) that can be implemented with hybrid PLL 100, accordingto an embodiment of the present disclosure. In one example, CCO 307 ofhybrid oscillator 300 of FIG. 3A and/or CCO 407 of hybrid oscillator 400of FIG. 4A can be implemented using the example of FIG. 4B. FIG. 4Billustrates a multi-core CCO 430 that combines low power oscillator core431 and high performance oscillator core 433. One difference between lowpower oscillator core 431 and high performance oscillator core 433 isthe channel length of each device in the ring cell. The ring cell can beconfigured by tri-state inverter to make sure the oscillator core can beshut down completely when oscillator core is disabled. In one example,the channel length for each inverter in the low power oscillator core431 can be 18 nm. In this example, the channel length for each inverterin the high performance oscillator core 433 can be 80 nm. Anotherexample of a multi-core CCO is discussed in U.S. Patent ApplicationPublication No. 2016-0072514, filed Sep. 26, 2014 and titled “DigitallyControlled Oscillator,” which is incorporated by reference in itsentirety. It is noted that multi-core CCO 430 of FIG. 4B is an exemplaryimplementation of a CCO and other implementations can also be used.

FIG. 5 is a block diagram of an exemplary hybrid PLL 500 according to anexemplary embodiment of the present disclosure. FIG. 5 illustrates amore detailed implementation of the hybrid PLL, such as hybrid PLL 100.Hybrid PLL 500 can be implemented using a frequency detector 511, adigital loop filter 513, an oscillator controller 515, a feedbackdivider 531, a phase frequency detector 533, a charge pump and analogfilter 535, and a hybrid oscillator 553.

As illustrated in FIG. 5, frequency detector 511 includes a referenceaccumulator 540, a variable accumulator 541, a summing element 542, aquality monitor 545, and a divider 546. Divider 546 receives referenceinput signal 501 and a frequency control code 547, which is the ratio ofthe desired frequency of output signal 551 divided by the frequency ofreference input signal 501. Reference accumulator 540 receives theoutput of divider 546 and generates a reference signal Rr, which is anaccumulation of the frequency control code 547 at an active edge of thereference input signal 501.

Variable accumulator 541 receives the output of divider 546 and outputsignal 551. Variable accumulator 541 increments a count on each activeedge of output signal 551, and generates a variable signal Rv. Thesumming element 542 determines the difference between the referencesignal Rr and the variable signal Rv to determine error signal 503. Insummary, frequency detector 511 is configured to convert the differencebetween reference input signal 501 and output signal 551 to a digitalcode (error signal 503.)

The error signal 503 is provided to digital loop filter 513. Digitalloop filter 513 controls a normalized tuning word NTW 505 depending onerror signal 503. Digital loop filter 513 includes a low-pass filter forattenuating unwanted spurs and phase noise at higher frequencies. Forexample, digital loop filter 513 scales down error signal 503 by 2″ togenerate normalized tuning word NTW 505. In one example normalizedtuning word NTW 505 can be a binary signal with signed 2's complement.

Oscillator controller 515 is implemented using a detector 561, a decoder562, a track trend detector 563, a track signal generator 564, a zerophase restart (ZPR) generator 565, and a system reset generator 566.Detector 561 is configured to receive normalized tuning word 505 andconvert normalized tuning word 505 to an oscillator tuning code OTW 567.For example, detector 561 normalizes the normalized tuning word NTW 505into the oscillator tuning code OTW 567 by multiplying the normalizedtuning word NTW 505 with a normalization value. Decoder 562 isconfigured to receive OTW 567 and convert OTW 567 to digital tuning word507 (such as a thermometer code, which is recognized by hybridoscillator 553).

In addition to generating digital tuning word 507, OTW 567 can be usedfor detecting tracking trends. In one example, track trend detector 563is configured to receive OTW 567 and is configured to determine when aswitch between frequency tracking mode and phase tracking mode should bemade. As discussed above and also in more detail below, the trackingmode detection can include a tracking from middle approach, a trenddetection approach, etc. If track trend detector 563 determines thathybrid PLL 500 is to operate in the frequency tracking mode using DCL110, track trend detector 563 triggers a signal 568 to tracking signalgenerator 564 to set tracking signal 509 at a first logical level (e.g.,low logical level.) Tracking signal 509 is provided, through aninverter, to charge pump and analog filter 535 and more specifically, tovoltage divider 571. When tracking signal 509 is at the first logicallevel (e.g., low logical level), voltage divider 571 is enabled and theoutput voltage of charge pump 573 and analog loop filer 572 is set to afixed voltage by voltage divider 571.

If track trend detector 563 determines that hybrid PLL 500 is to operatein the phase tracking mode using ACL 130, track trend detector 563triggers a signal 568 to tracking signal generator 564 to set trackingsignal 509 at a second logical level (e.g., high logical level.) Whentracking signal 509 is at the second logical level (e.g., high logicallevel), voltage divider 571 is turned off to release VCOIN 521. Inaddition, tracking signal 509 is input to detector 561. When trackingsignal 509 is at the second logical level (e.g., high logical level),detector 561 freezes OTW 567. Therefore, hybrid PLL 500 would operateusing ACL 130 for the phase tracking mode. During the phase trackingmode, hybrid PLL 500 operates as an analog PLL using, in part, phasefrequency detector 533 and divider 531. In one example, divider 531 caninclude a feedback divider 581 and an output divider 582.

During the frequency tracking mode where the operation of the hybrid PLL500 is performed using DCL 110 and hybrid oscillator 553, frequencydetector 511 converts the difference between reference input signal 501and output signal 551 to error signal 503. Digital loop filter 513controls a normalized tuning word NTW 505 depending on error signal 503.Since tracking signal 509 is at the first logical level (e.g., lowlogical level), detector 561 is enabled to convert normalized tuningword NTW 505 to oscillator tuning code OTW 567. Further, decoder 562 isconfigured to receive OTW 567 and convert OTW 567 to digital tuning word507. Digital tuning word 507 is used to control hybrid oscillator 553 totrack the frequency of reference input signal 501.

According to some embodiments, ZPR generator 565 of oscillatorcontroller 515 is configured to send a pulse to reference accumulator540 for aligning the output of reference accumulator 540 to the outputof variable accumulator when hybrid PLL 500 enters phase trackingoperation mode from frequency tracking operation mode. In one example,the pulse width of the pulse sent by ZPR generator 565 is about oneperiod of reference input signal 501. According to some embodiments,system reset generator 566 is configured to determine whether hybrid PLL500 needs to re-do frequency tracking. System reset generator 566 isconfigured to examine an LD signal, e.g., from phase frequency detector533. If the LD signal goes to a low logical level from a high logicallevel, it means that hybrid PLL 500 needs to re-do frequency tracking.Therefore, system reset generator 566 is configured to send a signal (inone example, an rstn_acc/rstn_sys signal with a pulse width of about oneperiod of reference input signal 501) to each block in hybrid-PLL. Theinput pin of “en_auto_rst” on system reset generator 566 can disable orenable this function.

Oscillator controller 515, and more specifically, track trend detector563 and track signal generator 564 are configured to track trends,determine a change in a trend, and change the logical level of trackingsignal 509. When tracking signal 509 is changed to a second logicallevel (e.g., high logical level), detector 561 will be disabled andhybrid PLL 500 will use ACL 130 to track the phase of the referenceinput signal. In the phase tracking mode, tracking signal 509 disablesvoltage divider 571 to release VCOIN 521. During the phase tracking modeof operation, ACL 130 and hybrid oscillator 553 can operate as an analogPLL. Output signal 551 (output from hybrid oscillator 553) is fed intodivider 581 of feedback divider 531. Output of feedback divider 531,which is output signal 551 divided by a value, is fed to phase frequencydetector 533. Phase frequency detector 533 is configured to detect thephase difference (and/or frequency difference) between feedback signal(output of feedback divider 581) and reference input signal 501. Phasefrequency detector 533 generates two output signals—UP signal and DNsignal. The pulse signals UP signal and DN signal are input to chargepump 573. Charge pump 573 converts the pulse signals UP signal and DNsignal to an UP/DN current. Analog filter 572 coverts the UP/DN currentoutput of the charge pump 573 into output voltage 521 (VCOIN). Outputvoltage 521 (VCOIN) is input to hybrid oscillator 553.

Traditionally, analog filters can occupy most of a PLL's area and thearea of the analog filter can be dominated by the oscillator's gain andthe PLL's bandwidth. Using a dual power analog PLL can reduce the sizeof the PLL compared to a single power analog PLL (in one example, thischange in area is about 47%.) The hybrid PLLs of the embodiments of thisdisclosure can reduce the area of the PLL compared to a dual poweranalog PLL. In one example, the change in area is about 40% compared todual power analog PLLs and about 70% compared to single power analogPLLs. This reduction in area in the hybrid PLLs of the embodiments ofthis disclosure are due, at least in part, to reduction in theoscillator's gain (because of the reduction in frequency range of theanalog controlled loop.) In other words, the hybrid PLLs of theembodiments of this disclosure partition a wide frequency range (e.g.,2.4 GHz) into, for example, 32 smaller frequency ranges. Since thefrequency range used by the analog controlled loop is smaller, theoscillator's gain is also reduced, which result in reduction of thehybrid PLL's area.

FIG. 6 depicts diagrams that illustrate trend detection, according tosome embodiments of the disclosure. In one example, the lockingprocedure of the hybrid PLL 100 and/or 500 can be described usingdiagram 600 and a trending procedure. In another example, the lockingprocedure of the hybrid PLL 100 and/or 500 can be described usingdiagrams 610 and 620.

In the locking procedure based on diagrams 600, the hybrid PLL 100and/or 500 starts the locking procedure from the lowest frequency (fbottom) of the frequency range of the hybrid PLL 100 and/or 500, wheredigital tuning word is zero. The hybrid PLL 100 and/or 500 increases thefrequency in steps until the hybrid PLL 100 and/or 500 reaches thecriteria of switching from frequency tracking mode to phase trackingmode. This criteria can be determined based on the trending mechanismdiscussed above. This locking procedure 600 can use the trend detectionprocess as discussed above and also in U.S. patent application Ser. No.15/135,212, filed Apr. 5, 2016 and titled “Automatic Detection of Changein PLL Locking Trend,” which is incorporated by reference in itsentirety and also summarized below with respect to diagram 620.

According to another example, in the locking procedure based on diagram610, hybrid PLL 100 and/or 500 starts the locking procedure from themiddle frequency of the frequency range of the PLL (e.g., middle digitaltuning word.) As a comparison, diagram 600 illustrates that the trackingtime is 11 reference cycles for a reference frequency of 3 GHz and is 4cycles for a reference frequency of 1.5 GHz. Diagram 610, on the otherhand, illustrates that the tracking time is 4 reference cycles for areference frequency of 3 GHz and 3 cycles for a reference frequency of1.5 GHz.

Diagram 620 illustrates an exemplary trend detection process inconnection with diagram 610 (e.g., tracking from middle.) In thisexample, oscillator controller 115 of FIG. 1 monitors error signal 103by analyzing signal 105 (which is derived from error signal 103) todetermine a trend, for example, a positive trend, a flat trend, and/or anegative trend in the error signal 103. As the frequency f_(OUT) and thefrequency f_(REF) converge, the error signal 103 decreases, and theerror signal 103 increases when the frequency f_(OUT) and the frequencyf_(REF) diverge. As shown in diagram 620, the error signal 103 can havea positive trend, for example, as the frequency f_(OUT) converges withthe frequency f_(REF). The error signal 103 can have a negative trend,for example, as the frequency f_(OUT) diverges from the frequencyf_(REF). The error signal 103 can have a flat trend, for example, whenthe frequency f_(OUT) is approximately proportional to the frequencyf_(REF). In accordance with an embodiment of the present disclosure,when the hybrid PLL 100 and/or 500 detects a change in trend (e.g., fromthe positive trend to the flat trend), oscillator controller 115 caninitiate the change in the tracking mode from frequency tracking usingthe DCL 110 to the phase tracking mode using the ACL 130.

FIG. 7 illustrates simulated waveforms, according to an embodiment ofthe disclosure. Graph 701 illustrates the output voltage 121 (VCOIN)that is input to hybrid oscillator 153 of FIG. 1. When the hybrid PLL100 and/or 500 is operating in the frequency tracking mode using thedigital control loop (shown as range 705 on FIG. 7), output voltage 121(VCOIN) is fixed. When the hybrid PLL 100 and/or 500 is operating in thephase tracking mode using the analog control loop (shown as range 707 onFIG. 7), output voltage 121 (VCOIN) controls hybrid oscillator 153. Inthis example, as shown in graph 701, hybrid PLL 100 and/or 500 locks tothe reference input signal after around 60 reference cycles (e.g., 1.2μs.)

Graph 703 illustrates the frequency of output signal 151 of FIG. 1. Thehybrid PLL 100 and/or 500 is operating in the frequency tracking modeusing the digital control loop in range 705, which takes around 0.2 μs.The hybrid PLL 100 and/or 500 is operating in the phase tracking modeusing the analog control loop in range 707, which takes around 1 μs. Thelocked frequency in this example is 2.9923 GHz.

Graph 710 illustrates a comparison of locking time between an ADPLL, ananalog PLL, and the hybrid PLL 100 and/or 500 of this disclosure. Asshown in bar 711, it takes analog PLL around 900 reference cycles tolock to the frequency and/or phase of the reference input signal. Bar713 illustrates that it take an ADPLL around 290 reference cycles tolock. For the hybrid PLL 100 and/or 500 of the embodiments of thisdisclosure, bar 715 shows that around 60 reference cycles are enough forlocking.

FIG. 8 is a flowchart illustrating an exemplary operational control flowof oscillator controller 115, according to an embodiment of thisdisclosure. Method 800 of FIG. 8 can be performed by processing logicthat can comprise hardware (e.g., circuitry, dedicated logic,programmable logic, microcode, etc.), software (e.g., instructionsexecuting on a processing device), or a combination thereof. It is to beappreciated that not all steps may be needed to perform the disclosureprovided herein. Further, some of the steps may be performedsimultaneously, or in a different order than shown in FIG. 8, as will beunderstood by a person of ordinary skill in the art.

Method 800 shall be described with reference to FIG. 1. However, method800 is not limited to that example embodiment. Also, the operation ofoscillator controller 115 is not limited to this operation control flowand other operational control flows are within the scope and spirit ofpresent disclosure.

In this exemplary method, the hybrid PLL 100 and/or 500 starts thelocking procedure from the middle frequency of the frequency range ofthe PLL. At 801, oscillator controller 115 determines the middlefrequency in the frequency range for the locking procedure. At 803,oscillator controller 115 (and more specifically, for example, tracktrend detector 563) monitors error signal 103 through analyzing signal105. As discussed above, signal 105 is derived from error signal 103going through digital loop filter 113. Monitoring error signal 103 caninclude collecting one or more samples of signal 105.

At 805, oscillator controller 115 (and more specifically, for example,track trend detector 563) determines the trend of the error signal 103based on the one or more collected samples from signal 105 to determinethe trend of error signal 103. At 807, oscillator controller 115continues to monitor error signal 103. For example, oscillatorcontroller 115 collects one or more additional samples of signal 105 anddetermines the trend of the error signal 103 for the additional samples.Oscillator controller 115 (and more specifically, for example, tracktrend detector 563) compares the new trend with the initial trend todetermine whether the trend of error signal 103 has changed. Method 800remains at 805 until oscillator controller 115 (and more specifically,for example, track trend detector 563) detects a change in the trend oferror signal 103. When the change is detected, the method moves to 809.

At 809, oscillator controller 115 can switch between digital controlledloop 110 and analog controlled loop 130 based on the detected change inthe trend of error signal 103. For example, oscillator controller 115configures hybrid PLL 100 to stop using DCL 110 (disables DCL 110 or atleast part of DCL 110) and configures hybrid PLL 100 to use ACL 130(enables ACL 130). Hybrid oscillator 153 will use the ACL 130 for thephase tracking mode of operation of hybrid PLL 100.

Similarly, oscillator controller 115 can configures hybrid PLL 100 tostop using ACL 130 (disables ACL 130) and configures hybrid PLL 100 touse DCL 110 (enables DCL 110 or at least part of DCL 110). Hybridoscillator 153 will use the DCL 110 for the frequency tracking mode ofoperation of hybrid PLL 100. For example, hybrid oscillator 153 iscontrolled using digital tuning word 107 and/or 507 generated by DCL110. The operational control flow can revert to 807 to continue tomonitor error signal 103 for other changes in the trend of error signal103.

The foregoing Detailed Description disclosed a hybrid PLL includes adigital controlled loop that receives a reference input signal and anoutput signal of the hybrid PLL, and generates a digital tuning word.The hybrid PLL further includes an analog controlled loop that receivesthe reference input signal and the output signal of the hybrid PLL, andgenerates an output voltage. The hybrid PLL also includes a hybridoscillator coupled to the digital controlled loop and the analogcontrolled loop. The digital controlled loop includes an oscillatorcontroller. The oscillator controller controls the hybrid oscillatorusing the digital tuning word and disables the analog controlled loopduring a frequency tracking operation mode of the hybrid PLL. Theoscillator controller enables the analog controlled loop to control thehybrid oscillator during the phase tracking operation mode of the hybridPLL.

The foregoing Detailed Description additionally disclosed a hybrid PLL,which includes a digital controlled loop that is implemented usingdigital components and operates during a frequency tracking mode. Thehybrid PLL also includes an analog controlled loop that is implementedusing analog components and operates during a phase tracking mode. Thehybrid PLL further includes an oscillator controller. The oscillatorcontroller receives an error signal, determines a trend of the errorsignal, and compares the trend of the error signal to a previous trendof the error signal. The oscillator controller further enables ordisables the analog controlled loop upon detecting a change in the trendof the error signal.

The foregoing Detailed Description additionally disclosed a method foroperating a hybrid phase lock loop (PLL). The method includes, during afrequency tracking operation mode of the hybrid PLL, controlling ahybrid oscillator using a digital tuning word generated by a digitalcontrolled loop and disabling an analog controlled loop. The methodfurther includes, during a phase tracking operation mode of the hybridPLL, enabling the analog controlled loop to control the hybridoscillator.

The hybrid PLL of the embodiments of this disclosure combine theadvantages of digital PLLs and analog PLLs. In other words, the hybridPLL of the embodiments of this disclosure combine the fast tracking andoscillator-gain reduction of the digital controlled loop with thecontinuous tuning mechanism and quantization noise free of the analogcontrolled loop. Further, the area of the hybrid PLL of the embodimentsof this disclosure can be smaller than traditional PLLs. Also, thehybrid PLL of the embodiments of this disclosure can improve the jittercontributed by, for example, power supply's noise. The hybrid PLL of theembodiments of this disclosure can also be placed at anywhere on a chipwithout a dedicated power supply. Accordingly the hybrid PLL of theembodiments of this disclosure can also be considered as anywhere PLLthat can operate in a low power consumption mode and/or a highperformance mode.

The foregoing disclosure outlines features of several embodiments sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A phase lock loop (PLL), comprising: a first loopconfigured to provide a digital output signal based on a comparison of areference input signal and an output signal of the PLL in a firstoperational mode of the PLL; a second loop configured to provide ananalog output signal based on a comparison of the reference input signaland a feedback signal that is proportional to the output signal in asecond operational mode of the PLL; and an oscillator coupled to thefirst loop and the second loop, configured to tune the output signal inaccordance with the digital output signal in the first operational modeof the PLL and to tune the output signal in accordance with the analogoutput signal in the second operational mode of the PLL, wherein thefirst loop comprises an oscillator controller configured to disable thesecond loop in the first operational mode of the PLL and to enable thesecond loop in the second operational mode of the PLL.
 2. The PLL ofclaim 1, wherein the first loop further comprises: a frequency detectorconfigured to compare the reference input signal and the output signalto provide a digital error signal, and wherein the oscillator controlleris further configured to provide a digital tuning word generated basedupon the digital error signal to the oscillator to tune the outputsignal.
 3. The PLL of claim 2, wherein the frequency detector and theoscillator controller are implemented using only digital components. 4.The PLL of claim 1, wherein the second loop comprises: a feedbackdivider configured to scale the output signal to provide the feedbacksignal; and a phase frequency detector configured to detect a differencein phase between the reference input signal and the feedback signal. 5.The PLL of claim 4, wherein the feedback divider and the phase frequencydetector are implemented using only analog components.
 6. The PLL ofclaim 1, wherein the oscillator controller is configured to provide atracking signal at a first logical level to disable the second loop inthe first operational mode of the PLL and to switch the tracking signalto a second logical level to enable the second loop in the secondoperational mode of the PLL upon detecting a change in a trend of adifference between the reference input signal and the output signal. 7.The PLL of claim 1, wherein the oscillator controller is furtherconfigured to: set the analog output signal to a fixed voltage in thefirst operational mode of the PLL, and adjust the analog output signalto track the reference input signal in the second operational mode ofthe PLL.
 8. A phase lock loop (PLL), comprising: a first loop,implemented using digital components, configured to provide a digitaloutput signal during a first operational mode to tune an output signalof the PLL; a second loop, implemented using analog components, andconfigured to provide an analog output signal during a secondoperational mode to tune the output signal; and an oscillator controllerconfigured to: disable the second loop in the first operational mode,receive an error signal generated based on a comparison of a referenceinput signal and the output signal in the first operational mode,compare a trend of the error signal to a previous trend of the errorsignal in the first operational mode, and enable the second loop upondetecting a change in the trend of the error signal to switch from thefirst operational mode to the second operational mode.
 9. The PLL ofclaim 8, wherein the first loop comprises: a frequency detectorconfigured to compare the reference input signal and the output signalto provide a digital error signal, and wherein the oscillator controlleris further configured to provide a digital tuning word generated basedupon the error signal to an oscillator to tune the output signal. 10.The PLL of claim 9, wherein the frequency detector and the oscillatorcontroller are implemented using only the digital components.
 11. ThePLL of claim 8, wherein the second loop comprises: a feedback dividerconfigured to scale the output signal to provide a feedback signal; anda phase frequency detector configured to detect a difference in phasebetween the reference input signal and the feedback signal.
 12. The PLLof claim 11, wherein the feedback divider and the phase frequencydetector are implemented using only the analog components.
 13. The PLLof claim 8, wherein the oscillator controller is configured to provide atracking signal at a first logical level to disable the second loop inthe first operational mode and to switch the tracking signal to a secondlogical level to enable the second loop in the second operational modeupon detecting the change in the trend of the error signal.
 14. The PLLof claim 8, wherein the oscillator controller is further configured to:set the analog output signal to a fixed voltage in the first operationalmode, and adjust the analog output signal to track the reference inputsignal in the second operational mode.
 15. A method for operating aphase lock loop (PLL), the method comprising: tuning an output signal ofthe PLL using a digital tuning word generated by a digital controlledloop during a frequency tracking operational mode of the PLL; receivingan error signal generated based on a comparison of a reference inputsignal and the output signal; comparing a trend of the error signal to aprevious trend of the error signal, and switching from the frequencytracking operational mode of the PLL to a phase tracking operationalmode to tune the output signal using an analog output voltage generatedby an analog controlled loop upon detecting a change in the trend of theerror signal.
 16. The method of claim 15, further comprising: comparingthe reference input signal and the output signal to provide a digitalerror signal, and generating the digital tuning word based upon thedigital error signal to an oscillator to tune the output signal.
 17. Themethod of claim 15, further comprising: scaling the output signal toprovide a feedback signal; and detecting a difference in phase betweenthe reference input signal and the feedback signal.
 18. The method ofclaim 15, further comprising: providing a tracking signal at a firstlogical level to disable the analog controlled loop in the frequencytracking operational mode, and wherein the switching comprises:switching the tracking signal to a second logical level to enable theanalog controlled loop in the phase tracking operational mode upondetecting the change in the trend of the error signal.
 19. The method ofclaim 15, further comprising: setting the analog output signal to afixed voltage in the frequency tracking operational mode, and adjustingthe analog output signal to track the reference input signal in thephase tracking operational mode.
 20. The method of claim 15, furthercomprising: disabling the analog controlled loop during the phasetracking operational mode.